Operational efficiency and the overall output of a gas turbine engine generally increases as the temperature of the hot combustion gas stream increases. High combustion gas stream temperatures, however, may produce higher levels of nitrogen oxides and other types of regulated emissions. A balancing act thus exists between the benefits of operating the gas turbine engine in an efficient high temperature range while also ensuring that the output of nitrogen oxides and other types of regulated emissions remain below mandated levels. Moreover, varying load levels, varying ambient conditions, and many other types of operational parameters also may have a significant impact on overall gas turbine efficiency and emissions.
Lower emission levels of nitrogen oxides and the like may be promoted by providing for good mixing of the fuel stream and the air stream prior to combustion. Such premixing tends to reduce combustion temperature gradients and the output of nitrogen oxides. One method of providing such good mixing is through the use of a combustor with a number of micro-mixer fuel nozzles. Generally described, a micro-mixer fuel nozzle mixes small volumes of the fuel and the air in a number of micro-mixer tubes within a plenum before combustion.
Although current micro-mixer combustors and micro-mixer fuel nozzle designs provide improved combustion performance, the operability window for a micro-mixer fuel nozzle in certain types of operating conditions may be defined at least partially by concerns with dynamics and emissions. Specifically, the operating frequencies of certain internal components may couple so as to create a high or a low frequency dynamics field. Such a dynamics field may have a negative impact on the physical properties of the combustor components as well as the downstream turbine components. Given such, current combustor designs may attempt to avoid such operating conditions by staging the flows of fuel or air to prevent the formation of a dynamics field. Staging seeks to create local zones of stable combustion even if the bulk conditions may place the design outside of typical operating limits in terms of emissions, flammability, and the like. Such staging, however, may require time intensive calibration and also may require operation at less than optimum levels.
There is thus a desire for improved micro-mixer combustor designs. Such improved micro-mixer combustor designs may promote good mixing of the flows of fuel and air therein so as to operate at higher temperatures and efficiency but with lower overall emissions and lower dynamics. Moreover, such improved micro-mixer combustor designs may accomplish these goals without greatly increasing overall system complexity and costs.